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    * {
      margin: 0;
      padding: 0;
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    .box {
      display: flex;
    }

    .left {
      width: 300px;
      height: 500px;
      background-color: pink;
    }

    .main {
      flex: 1;
      height: 500px;
      background-color: skyblue;
    }

    @media (max-width: 768px) {
      .left {
        display: none;
      }
    }
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  <div class="box">
    <div class="left">left</div>
    <div class="main">
      响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果响应式网页效果
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